Our proposed research is multiphasic and includes: The effect of artificial cardiac pacing on control of bradycardis and tachycardias; the influence of drugs on threshold to pace, cardiac output and intracardiac voltages; the clinical course of paced patients, including underlying rhythm, pacemaker modification, morbidity and mortality; the functional reliability and utility of existent pacing equipment and development of new equipment of improved flexibility, utility, reliability and longevity toward the "ideal pacemaker"; definition of causes of malfunction; electrical, electronic, magnetic, unusual intracardiac currents, analyze the intracardiac electrograms and their influence on pacemaker triggering; the utility of electronic "clinic" and transtelephone methods of monitoring to set up a rapid computer record keeping, retrieval and analysis system for the massive clinical, electronic and physiologic data we develop; to develop non-invasive techniques of measuring pacemaker function and cardiac response. Our objectives are: 1) To make pacemakers flexible, reliable, long lasting; 2) To prolong life in as near physiologic function as possible; 3) To understand better the interrelationship of pacing and the patient; 4) To set standards of safety and performance. The first years will emphasize computer analysis of the data of 2000 patients, tests of new programmable and nuclear pacemakers and life extension of pacer systems.